Amines are commonly used as epoxy curing agents for heat cured structural composite and adhesive applications. The class of amine used as a curing agent is vital to achieve the final performance of the cured product. Each of the amine classes (primary, secondary, or tertiary amine) is cured at a specific temperature in order to achieve full cure. The cure temperature dictates the final service temperature, which is indicated by its Tg. In general, epoxy resins are predominately cured with primary and secondary amines. Tertiary amines are generally used as co-curing agents or catalysts in combination with primary and secondary amines.
Tertiary amines are known to induce homopolymerization of epoxy during cure which results in lower mechanical properties due to embrittlement. Tertiary amines such as benzyl dimethylamine (BDMA), 2,4,6-Tris-(dimethylaminomethyl)phenol (Ancamine® K-54), and mon-dimethylaminomethyl phenol (A1110®) are not suitable for use as a major curing agent to cure epoxy, because they induce homopolymerization which results in lower mechanical properties due to embrittlement and high exotherm during cure.
Epoxy resin systems are used in the manufacturing of various structural parts, including composites and adhesives. Examples of articles that are being evaluated for manufacturing from epoxy resin systems include composite pipes, pressure vessels, automotive parts and windmill blades. Fabricating such parts includes a number of requirements for effective manufacturing especially when complex manufacturing processes are used. These processes include but are not limited to resin infusion, resin transfer molding, filament winding and large casting. One need in the art is for reduced exothermic heat release during the epoxy resin system cure of the article (composite) in thicker sections of the article, since in such sections, the exothermic heat released during cure cannot be easily conducted away from the article. If excessive temperatures are reached during the cure process, thermal degradation of the cured resin in the “hot spots” can occur with resultant mechanical property loss in the fabricated parts.
Additionally, during cure, the composite parts may undergo thermal shrinkage. Thermal shrinkage of a cured epoxy resin causes stresses to build up in a composite during cool down from the maximum temperature reached at or after gelation. The stresses sometimes lead to interlaminar cracking in the article, with resultant loss of mechanical properties. The higher the temperature reached during cure after the gel point, the greater the amount of stress that will accumulate in the article during cooling.
Standard epoxy systems for fabricating structural parts are cured with stoichiometric quantities of aliphatic amines, usually primary amines. The systems generally have high cure exothermic temperatures, with the center of a 100-gram mass of resin/curing agent mixture (contained within a three inch diameter cylinder) often reaching a peak temperature of 250° C. or higher when cured in a 70° C. oven. Alternatively, epoxy systems cured with anhydride-based curing agents may often have lower cure exothermic heat release than those cured with primary amines. However, anhydride-cured systems typically require higher mold temperatures than systems cured with primary aliphatic amines to reach an acceptable degree of cure and level of cured properties.
Other requirements in the art include the absence of highly volatile components in the system for elevated temperature cure. The emission of volatile compounds during processing creates unwanted environmental, health and safety considerations.
Systems for composite processing require an initial mixed viscosity low enough (and rate of viscosity increase at the impregnation temperature low enough) to enable the reinforcing fiber preform to be completely wet with resin before the resin system becomes too viscous for satisfactory flow through the fibers and fabric of the substrate. The requirement for low initial viscosity and long pot life becomes more stringent as the size of the composite part increases.
In light of the above, there is a need in the art for improved curing agents for producing epoxy resin systems which have reduced exothermic heat release combined with desired cured mechanical properties when compared to the prior art resin compositions. Such curing agents must be free of undesirable features such as volatile emissions.